As one of the conventional holographic recording methods of this type, an angle multiplex holographic recording method is known in which the incident angle of a reference beam with respect to a holographic recording medium is modulated while an object beam is fixed. In this case, the beam shapes of the object beam and the reference beam are both circular.
In a holographic recording medium in which angle multiplex recording has been performed as mentioned above, when content associative reproduction (search) is performed, a technique known as associative recording is utilized for instantly searching required information from the information multiplex-recorded in the same position in the holographic recording medium.
For example as described in 10 Nov. 1999/Vol. 38, No. 32/APPLIED OPTICS p6779-6784, by projecting only an object beam onto a holographic recording medium on which information has been recorded to thereby allow the object beam to display the data desired to be reproduced or a part thereof, a diffraction beam of the object beam is emitted in the direction same as that of a reference beam upon recording according to the correlation with the data desired to be reproduced from a lot of data pages recorded on the holographic recording medium. This diffraction beam is detected by an address detector constituted by a plurality of CCDs or imaging elements or a photo detector array.
If the diffraction beam of the object beam is detected by the address detector as mentioned above, the photo detector array or the like constituting the address detector must separately detect the incident diffraction beams according to the spacing of the incident angle modulation of the reference beam upon recording so as not to cause mutual crosstalk.
Generally, the spacing of the incident angle of the reference beam upon recording or the modulation angle spacing is often set in the range of from 10 millidegrees to 1 degree.
If, for example, a beam diameter of the reference beam of 0.3 mm and a modulation spacing of the incident angle of 100 millidegrees are employed, the distance of the address detector with respect to a holographic recording medium must be set to approximately 20 cm in order to separate beam spots on the address detector so as to prevent mutual overlapping. If the modulation angle spacing is reduced in order to increase the recording capacity of a holographic recording medium, the address detector must be located at a distance further away from the holographic recording medium for preventing the overlapping of the beam diameters on the address detector. Thus, a problem arises that the volume of an apparatus increases.
Moreover, if the beam diameters of the reference beam and the object beam are reduced, the utilization efficiency of the reference beam is lowered, causing new problems that a recording rate and the contrast of interference fringes are lowered.